What about the Borg? Judging from the Star Trek movies, they seem better at the starship-boarding and raygun-fighting thing than your average red-shirted security guard or away team member.
In many movies, even the unnamed among both the Nazis and Mafiosi come across as people with whom it would be best not to trifle. Also, in many of the movies I watch, these folks do their jobs very well, which is why the Good Guys must frequently put together elite teams or recruit outside help to put the Bad Guys out of business.
Although they often seem kind of clownish as individuals, the post-apocalypse gangs that terrorize Australia in the Mad Max movies are composed of a bunch of characters I would hate to fight or race against. I wouldn’t hire those guys for any project that took a lot of organisation, but as terrorists of the wasteland they do a pretty good job (until Mel Gibson shows up).
Longshanks’ armies, when commanded by the king himself, in Braveheart. Did very well… though of course they did have superior weapons, horses, armaments and a willingness to cut down their own troops in combat with arrows, along with the enemy. That’s damned ruthless but effective.
Oh! and the Orc armies’ attack on Helm’s Deep in Lord Of The Rings: The Two Towers. They overwhelmed the joint if not for that damned Gandalf and the Ents…
Magneto’s brotherhood of mutants did pretty well in the first X-men movie. They were very effective in separating each member of the X-men and exploiting their weaknesses. Toad in particular was remarkable considering his rather lackluster powers.
Assuming we’re allowed to use characters from books, I nominate the Cauldron-Born from Lloyd Alexander’s Prydain series. Can’t get much better than a legion of invincible zombies.
Before they got massacred, the Dragonslayers from Vision of Escaflowene did pretty well. Would have done even better if they didn’t have to spend half their time saving thier nutcase commander. Even after they got massacred, they did pretty good, actually.
The Zentradi from Macross/Robotech gave a good accounting of themselves as a force, though one-on-one they were a little unreliable.
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‘How can they keep missing this huge, slow moving white thing?’